Today, cellular operators are tasked with utilizing more spectrum to address the increasing demand for mobile data. Cellular operators are increasingly interested in the use of Long Term Evolution (LTE) on an unlicensed spectrum (i.e., LTE-U or license assisted access (LAA)) due to the availability of large amounts of bandwidth.
A distributed antennas system (DAS) is a way to deal with isolated spots of poor coverage inside a large building by installing several relatively small antennas throughout the building. The antennas are physically connected to a central controller, which is connected to the wireless carrier network's base station. Typically, in systems that utilize a number of antennas, the same signal is sent from all of the antennas without taking into account antenna deployment and the position of the wireless communication device (WCD) (also referred to as user equipment (UE)) for which the signal is intended. When a DAS deployment with LTE is operating on unlicensed frequencies, it is further required that, before a signal is transmitted, a Clear Channel Assessment (CCA) is performed to allow transmission from a specific antenna. That is, an antenna of a DAS is used to transmit a signal only when the antenna has received an indication that the channel on which the antenna is configured to transmit the signal (the “operating channel”) is clear (e.g., no other system is using the channel or the channel is free of noise). Such a clear channel indication is referred to as a “successful CCA.” Antennas without a successful CCA are muted (e.g., not used to transmit the signal). CCA involves a receiver detecting energy (e.g., noise or traffic) in the operating channel and backing off a data transmission when, for example, traffic or excessive noise is detected (i.e., a clear channel is not indicated), thereby avoiding interference.
Unlicensed spectrums allow mobile operators to boost coverage in their cellular networks by using the unlicensed band already populated by Wi-Fi devices. While the operators ordinarily rely on the radio spectrum to which they have exclusive licenses, LTE-U shares space with Wi-Fi equipment such as smartphones and laptops already using that band.
If scheduling of a UE is performed without taking into account a position of the UE within the DAS, a number of disadvantageous and unwanted scenarios will occur. For example, in a traditional DAS, operating on a licensed LTE Band (e.g., no CCA required for an individual antenna to be allowed to transmit) a major drawback is that a UE will potentially be scheduled using antennas which do not provide coverage at the position where the UE is located. Such transmissions do not improve the radio link for that particular UE, but instead, contributes to an increase in radio network interference, which adversely impacts network capacity and performance.
In another example, the CCA requirement used in the unlicensed spectrum adds complexity to the network. For example, in the most basic solution in unlicensed spectrums, a certain UE is scheduled as soon as any antenna in the system has received a successful CCA. However, since there is no knowledge regarding whether a particular UE is in the coverage area of the antenna(s) which have a successful CCA, there is a probability that the UE will not be reached by the transmission, triggering retransmissions, and in a worst case, eventually leading to stalling and radio link failure. These rudimentary solutions put very high and unwanted requirements on the actual physical installation of the DAS.
Furthermore, in the DAS, it is not feasible to have one or a few antennas in the system where the radio environment differs too much from the rest of the antennas. For example, in a case where one antenna is installed in a basement while the rest are installed on higher floors in a building (FIG. 1(A)), the radio environment may be good (triggering successful CCA) in the basement while the radio environment higher up in the building will be worse. This scenario results in that UEs on higher floors will be scheduled only on an antenna that cannot be heard by the UE leading to the unwanted effects previously discussed. A similar situation arises in a corridor (FIG. 1(B)) where radio conditions may vary at different parts of the corridor.